Mirror telescopes have aplanatic mirror systems and are sometimes referred to as coma-free reflecting telescopes. They are used primarily for astro-photographic purposes and are characterized by satisfying the sine condition whereby a larger image field is coma-free. A concave mirror or a composite mirror system functions as an objective in mirror telescopes. The primary mirror is configured as a parabolic mirror to avoid spherical aberration. An important advantage of the mirror telescopes is the absence of chromatic aberration. Mirror telescopes can be produced with far greater objective diameters than refracting telescopes and the aperture ratio can be substantially larger because of the absence of the residual defects on the optical axis. Such residual defects include secondary spectrum and aperture aberrations.
A disadvantage of the mirror telescopes is their high sensitivity with respect to adjustment. This is especially the case when the optical quality must be maintained over several years without refocusing and be independent of temperature changes, vibration load and material aging. These requirement apply, for example, to the use in unaccessible assembly groups such as in satellites.